Mineral oils (also half- and full-synthetic) as well as animal oils and vegetable oils, with or without additives, are used widely in industry for various purposes, such as lubrication, cooling and insulation. During such use the oils commonly become contaminated by different kinds of particles. Depending on the composition and the particular use different methods for regeneration of contaminated oils are used.
Historically, contaminated oils have been filtered in filter beds containing clay, bleaching earth or kieselguhr.
Purification of mineral oils from suspended particles is described for example in U.S. Pat. No. 4,094,770. According to this patent the particles that are not filtrated away are removed by addition of an agglomerating aid in the form of a mixture of acetone and 2-butanone. The agglomerated particles settle and may be removed from the oil. A finishing distillation step is necessary in order to purify the oil from the agglomerating aid.
U.S. Pat. No. 4,491,515 describes purification of lubricating oil which has been used in vehicles. The oil, which may contain many kinds of contaminants, is purified by addition of a compound containing a carbonyl group (ketone) mixed with a water-containing electrolyte, for example an inorganic or organic acid. When this acid has been added to the oil, there is relatively rapidly obtained an agglomeration of particulate contaminants that may be removed by settling or centrifugation. The ketone is recovered in a distillation step.
U.S. Pat. No. 4,519,899 describes purification of rolling oil which has been contaminated with particles of the material which has been treated at the rolling operation. A coagulating agent, as for example a water-containing soda solution, is mixed with the oil in a carefully controlled amount. The particles coagulate and are found in the water phase, which may be removed by settling or in a centrifugal separator.
In SE 512 750 there is described a method for gravimetric separation of an oil which is contaminated with particles and/or water. According to this method a collection polymer or polymer mixture, which is not soluble in oil, is added to the contaminated oil and mixed with the same, after which separation of the oil and the collection polymer occurs. The collection polymer and the main part of the contaminants form a bottom phase, while the oil forms a top phase. The bottom phase with the collection polymer and the contaminants is removed. According to this publication the separation may take place by centrifugation.
The contaminants in the shape of particles, which are to be removed from the oil, may in many cases be very small and difficult to remove from the oil. The amount of particles may also be small. The amount of separating aid that must be added may therefore be comparatively small. To separate two liquid phases of different density from each other in a centrifugal separator may cause problem if one of the phases is present only in a small amount.